Polyethylene has been used in the production of various film products, such as bags and packaging. Examples of such products include shipping sack applications, fertilizer bags, insulation bags, food packaging, lamination film etc.
Biaxially-oriented, blown polyolefin films are generally known in the art and have been used in the production of articles such as garbage bags, shopping bags, food wraps, and any number of articles requiring polymer chain orientation in both the machine direction (MD) and the transverse direction (TD) of the film. Although cast films may be processed to achieve biaxial-orientation, blown films are generally preferred as they usually require less subsequent processing steps to achieve good mechanical properties. Desirable mechanical properties include dart impact, tear strength in both the machine and transverse directions, tensile strength in both the machine and transverse directions, elastic modulus, slow puncture resistance etc. Optical properties that are required namely transparency are measured under gloss and haze.
Tailoring the properties of polyolefins, such as polyethylene, to fit a desired applicability is constantly ongoing. In this case particularly, the objective is to have a better balance between mechanical and optical properties.
Metallocene catalyzed polyethylene having high and medium densities are known to have good optical properties. However, for film applications, they have mechanical properties which can still be improved, in particular dart impact, tear strength and slow puncture resistance. On the other hand, for example, polyethylene prepared with dual site catalysts in the gas phase or with Ziegler-Natta catalysts have good mechanical properties, but poorer optical properties. Nucleating agents are required to improve the gloss and haze. However, nucleating agents are not particularly effective for polyethylene resins. For example, for a haze of 30%, a nucleating agent cannot improve haze to less than 25%.
Achieving outstanding mechanical properties such as dart impact and/or slow puncture resistance and/or tear strength, as well as good optical properties such as haze and gloss, is an objective of the present invention.
Another objective is to also maintain good processability of the polyethylene resin compositions i.e. a high melt strength, to provide a polyethylene resin composition particularly suitable for film applications.